It's really, really rude and unpleasant to flaunt your greater knowledge of the groom's life - and previous loves - to the bride at any time, let alone at her wedding to the guy.
Like.. if you saw the woman pre-church, and the back of her dress was tucked in her knickers, you'd tell her before she went up the aisle. Because otherwise, though she may not herself notice at all, the rest of the guests would ALL be seeing her looking ridiculous on her wedding day, which one would assume would be kind of a big deal day for her. This is like not telling her and letting her flash her arse while pledging herself to someone in all ways, only deeper.